Method for the production of a particle-containing preparation of tetrahydro-3,5-dimethyl-1,3,5-thiadiazin-2-thione

ABSTRACT

Described is a process for the preparation of a particulate tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione product by combining a first aqueous solution comprising methylammonium N-methyldithiocarbamate with a second aqueous solution comprising formaldehyde, followed by separation and drying of the resulting solid, which comprises combining the first and the second aqueous solutions in such a way that the ratio between the concentrations of dithiocarbamate functions and of formaldehyde is essentially constant in the reaction mixture over time during the duration of the reaction.

[0001] The present invention relates to a process for the preparation ofa particulate tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione productby combining a first aqueous solution comprising methylammoniumN-methyldithiocarbamate with a second aqueous solution comprisingformaldehyde, followed by separation and drying of the resulting solid.

[0002] Tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione, which is alsoknown under the common name dazomet, is employed in agriculture andhorticulture for soil sterilization, i.e. for controlling nematodes,germinating plants and cell fungi (cf. U.S. Pat. No. 2,838,389). Theaction is based on the slow release of methyl isothiocyanate in the soilby hydrolytic and/or enzymatic breakdown oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione.

[0003] To avoid the formation of aerosols, which are a potential healthhazard, during packaging, handling and/or applying the activeingredient, a particulatetetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione product is desirablewhose fines, with particle sizes of less than 100 μm, amount to aslittle as possible. On the other hand, the particulate product shouldalso not contain a substantial proportion of coarse particles withparticle sizes of over 400 μm in order to guarantee sufficiently rapiddecomposition of the active ingredient in the soil. The knownpreparation processes oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione only allow thepreparation of particulatetetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione products with aninadequately homogeneous particle size distribution.

[0004] WO 93/13085 describes a process for the preparation oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione granules by reactingthe methylammonium salt of N-methyldithiocarbamic acid with formaldehydein the presence of a diaminoalkylene. Under the reaction conditions, thediaminoalkylene leads to the formation of products which act ascrystallization inhibitors and which, together with the crystallites ofthe active ingredient, form a random conglomerate. WO 93/13085recommends adding the methylammonium N-methyldithiocarbamate solution toan aqueous formaldehyde solution. It has emerged that reproducibleparticle size distributions can only be obtained by the known processwhen a large number of parameters, including the rate at which thereactants are added, the intensity of mixing the reactants, the mixingtime and the like, are observed accurately. The preparation of a productwith constant properties and of flexible response to varying demands isthus made difficult.

[0005] It is an object of the present invention to provide a process forthe preparation of a particulatetetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione product which,despite simple process control, yields products with a narrow particlesize distribution, in particular with a reduced content of fines of aparticle size of less than 100μm.

[0006] This object is achieved according to the invention by a processfor the preparation of a particulatetetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione product by combininga first aqueous solution comprising methylammoniumN-methyldithiocarbamate with a second aqueous solution comprisingformaldehyde, followed by separation and drying of the resulting solid,which comprises combining the first and the second aqueous solutions insuch a way that the ratio between the concentrations of dithiocarbamatefunctions and of formaldehyde is essentially constant in the reactionmixture over time.

[0007] The invention furthermore relates to a particulate agrotechnicalproduct with a tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thionecontent of at least 95% by weight and such a particle size distributionthat less than 7% by weight, preferably less than 3% by weight, of theparticles have a particle diameter of less than 100 μm, over 50% byweight of the particles have a particle diameter of less than 200 μm,over 90% by weight of the particles have a particle diameter of lessthan 300 μm, and over 95% by weight of the particles have a particlediameter of less than 400 μm.

[0008] The particulate product obtained in accordance with the inventionpreferably has a bulk density of 0.4 to 0.8 kg/l, in particular 0.6 to0.7 kg/l.

[0009] It has been found that narrow particle size distributions can beobtained when the first and the second aqueous solutions are combined insuch a way that the ratio between the molar concentration ofdithiocarbamate function and formaldehyde in the reaction mixture isessentially constant over the duration of the reaction.

[0010] The process according to the invention thus differs essentiallyfrom the known processes, in which a reactant, as a rule the aqueousformaldehyde solution, is introduced into the reaction vessel and theother reactant is metered in over a period of time. It can be seen that,in the known processes, the reactant which has initially been introducedis present in a multiple molar excess at the beginning of themetering-in operation. The ratio between the reactant which hasinitially been introduced and the reactant which is metered in thendecreases constantly over the duration of the metering-in operation.

[0011] The process according to the invention can be carried outsemicontinuously or continuously. To this end, it is expedient tointroduce essentially stoichiometrically equivalent amounts of the firstaqueous solution, calculated as dithiocarbamate functions, and of thesecond aqueous solution, calculated as formaldehyde, i.e. essentiallytwice the molar amount of formaldehyde, into a reaction space per unittime. An “essentially stoichiometrically equivalent” amount is such anamount which is within 20%, preferably within 10%, of thestoichiometrically required amount of the reactant in question. It isalso possible to introduce an amount greater than the stoichiometricallyrequired amount of a reactant if the accumulation, in the reactionmixture, of the reactant employed in excess is prevented by suitablemeasures. For example, the accumulation can be prevented by continuouslyremoving the excess, for example by continuously discarding some of themother liquor which is obtained when the product is removed from thereaction mixture, as is illustrated in further detail hereinbelow.

[0012] Reactors which are suitable for carrying out the reactionprocedure continuously are customary reactors such as, in particular, acontinuous stirred-vessel reactor or a stirred-vessel cascade. It isexpedient to ensure good mixing of the reactants in the reaction space.The introduction of the first and/or second aqueous reaction solutioncan be carried out for example in such a way that some of the reactionmixture is continuously removed from the reaction space, mixed with thefirst and/or second aqueous reaction solution and recirculated into thereaction space. Removal, mixing and recirculating are effected forexample by pumping the reaction mixture via a metering and mixingsection into which the first and/or second solution are fed. Instead ofstirred-vessel reactors or stirred-vessel cascades, tubular reactorswhich are optionally provided with elements like static mixers may alsobe used.

[0013] Sparingly solubletetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione is formed as aqueoussuspension when the aqueous solutions are combined. When the processaccording to the invention is carried out continuously, suspension iscontinuously withdrawn from the reaction space or, when using a reactorcascade, from the last reactor of the cascade. The solid is separatedfrom the resulting suspension by customary processes, for example byfiltration or centrifugation. Suitable devices, such as pressurefilters, vacuum filter belts, rotary drum type filters and centrifuges,are known to those skilled in the art. All or some of the mother liquorwhich remains can be recirculated or eliminated from the process. Anaccumulation of contaminants or of reactants employed in excess can beprevented in the system by discharging at least some of the motherliquor.

[0014] The solid which has been separated off from the mother liquor canbe washed, for example with cold or warm water. To this end, the solidcan be made into a slurry with the wash medium and subsequently beseparated off.

[0015] The solid which has been separated off from the mother liquorand, if appropriate, washed can then be dried by customary methods.Pneumatic conveyor dryers or fluidized beds are suitable for thispurpose.

[0016] In some cases, it may be advantageous to mix the moist cakeobtained after the mother liquor has been removed with material whichhas already been dried and to subject the mixture to further drying inorder to prevent caking during drying. When the process is carried outcontinuously, this can be achieved by recirculating some of the driedmaterial.

[0017] It is preferred to combine the first and second aqueous solutionsin the presence of seed crystals oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione. The addition of seedcrystals additionally allows the influencing of the particle sizedistribution and/or bulk density of the resulting particulate product.The material used as seed crystals is finely dividedtetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione, for example in anamount of 1.5 mol % to 10 mol %, preferably 2.5 mol % to 7.5 mol %, inparticular 3 mol % to 6 mol %, based on the methylammoniumN-methyldithiocarbamate employed. Seed crystals with a particle size ofless than 100 μm are preferably used. Usually, 90% of the seed crystalsshould have a particle size of between 50 and 5 μm. Particularlypreferred is a particle size distribution of the seed crystals in which100% of the particles are smaller than 100 μm, approximately 90% arebetween 50 and 1 μm and approximately 10% are less than 5 μm.

[0018] To achieve as uniform as possible a distribution of the seedcrystals in the reaction mixture, the seed crystals are preferably addedto the reaction mixture in the form of an aqueous suspension.

[0019] Seed crystals of a desired particle size can be obtained bycomminuting, for example grinding,tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione which has beenprepared beforehand. In a possible embodiment of the invention, seedcrystals of a suitable size are obtained by subjecting the particulatesolid removed from the reaction space to sizing, resulting in fines andin course particles, the fines being recirculated into the reactionspace as seed crystals and the coarse particles being discharged fromthe process as product. Sizing can be carried out both in the suspendedstate and in the dry state. To carry out the sizing operation in the drystate, the solid is beforehand separated from the mother liquor anddried. The fines obtained can be suspended in an aqueous medium to berecirculated into the reaction space. Suitable devices for sizing are,for example, hydrocyclones and wet screens for screening suspendedparticles, and cyclones, sieves or screens for screening driedparticles.

[0020] To prepare the first aqueous solution, a procedure is generallyfollowed in which an aqueous solution of methylamine, if appropriatewith the concomitant use of an alkylenediamine such as describedhereinbelow, is first treated with carbon disulfide. The reaction of theaqueous methylamine solution, if appropriate with the concomitant use ofan alkylenediamine, with carbon disulfide can be carried outcontinuously, semicontinuously or batchwise. The continuous reaction canbe carried out in any reactors which are suitable for this purpose, forexample reaction towers or jet-loop reactors, preferably in stirredvessels. To achieve as complete reaction as possible, it is recommendedto use a cascade of reactors composed of at least one main reactor and asecondary reactor.

[0021] To prepare the first aqueous solution, it is preferred to use anexcess of carbon disulfide. Unreacted carbon disulfide separates fromthe aqueous solution of the dithiocarbamate, being the phase with thegreater specific weight. It has proved advantageous to remove theunreacted carbon disulfide to less than 0.5% by weight, and particularlyless than 0.3% by weight, especially preferably less than 0.1% byweight, based on the first aqueous solution, before combining the latterwith the second aqueous solution. Greater amounts of excess carbondisulfide tend to form finely dispersed droplets which interfere withthe precipitation process in the subsequent reaction with the aqueousformaldehyde solution owing to flotation and which, undesirably, canlead to the increased formation of ultrafine particles with a size ofless than, for example, 10 μm. Separation of the unreacted carbondisulfide can be achieved for example by settling and subsequent phaseseparation. For settling, the mixture of aqueous carbamate solution andunreacted carbon disulfide can be passed into a calming zone. This isexpediently effected in a continuously operated phase separation vesselin horizontal position through which a slow flow is passed. Owing to thedifferent densities of the phases, the emulsion separates in the fieldof gravitation so that the two phases are present coherently andessentially without foreign phases one above the other.

[0022] A disadvantage of largely removing the carbon disulfide phase arethe very long settling times. In order to achieve more rapid phaseseparation, one or more coalescence levels with an integrated phaseseparation device or a phase separation device which is arrangeddownstream are advantageously used. Generally suitable are separatorswith coalescence devices such as packing material, coalescence faces orfine-celled elements over which or through which the emulsion to beseparated must flow. If appropriate, most of the unreacted carbondisulfide is first removed, and the aqueous solution, which stillcomprises finely dispersed carbon disulfide droplets, is passed throughan apparatus with coalescence-promoting devices and the coalesced carbondisulfide phase is separated off.

[0023] As a rule, the devices with coalescence phases are stacks ofsheets with corrugated or oblique faces to which dispersed dropletsadhere and initially form a film. When this film covers the individualsheet and is sufficiently thick, large drops of the dispersed phase format the edge of the sheet and drop downward. They subsequently form alayer in the separator which can be separated readily by mechanicalmeans. In the case of fine-celled devices, the internal structure of theelements forces the finely dispersed drops to come into contact with theinternal surface and they then form a film and leave the hollowstructure of the fine-celled elements as combined larger drops. Packingmaterial which is suitable is packing material conventionally used indistillation. The emulsion to be separated is preferably passed througha bed of packing material. Wetting of the large surface of the packagingmaterial results in surface coalescence and simultaneously drop-to-dropcoalescence, owing to the movement of the drops.

[0024] Porous devices in the form of filter cartridges composed of, forexample, porous polypropylene, have proved advantageous. A mean poresize of the porous devices of less than 50 μm is particularly suitable.

[0025] Since not only the reaction of carbon disulfide with methylamine,but also the reaction of methylammonium N-methyldithiocarbamate withformaldehyde, is exothermic, while the intermediate and the product aresensitive to high temperatures, it is recommended to carry out thereaction with cooling. In general, the reactions proceed at a sufficientrate above 10° C., while temperatures above 50° C. increasingly lead tothe formation of undesired byproducts. The reactions are thereforeusually carried out at a temperature of 20 to 40° C.

[0026] In preferred embodiments of the process according to theinvention, the first aqueous solution comprises, in addition tomethylammonium N-methylthiocarbamate, at least one alkylenediamineand/or reaction products thereof with carbon disulfide. The firstaqueous solution expediently comprises 0.1 to 10 mol %, preferably 0.2to 5 mol %, in particular 0.5 to 1.5 mol %, of alkylenediamine, based onthe amount of methylamine on which it is based. Suitablealkylenediamines have the formula I

R¹—NH-A-NH—R²  (I)

[0027] in which R¹ and R² independently of one another are hydrogen orthe compound C₁-C₄-alkyl and A is straight-chain or branchedC₂-C₈-alkylene, preferably 1,2-ethylene, 1,2-propylene, 1,3-propylene or1,4-butylene.

[0028] Preferred alkylenediamines are those mentioned in WO 93/13085.Preferred among these are ethylenediamine,1-(N-methylamino)-2-aminoethane, 1,2-di(N-methylamino)ethane,1,2-diaminopropane, 1,2-di(N-methylamino)propane and1-(N-methylamino)-2-aminopropane. Ethylendiamine is especiallypreferred. The pure compounds, but also mixtures of these compounds, maybe employed.

[0029] The use of alkylenediamines with two primary amino groups, forexample those of the above formula I in which R¹═R²═H, in particularethylenediamine, is preferred. It has emerged that particularlyadvantageous particle size distributions are obtained when thealkylenediamine and its possible reaction products with carbondisulfide, i.e. its reaction product with 1 mol of carbon disulfide(N-aminoalkyldithiocarbamate) and its reaction product with 2 moles ofcarbon disulfide (alkylene-N,N′-bis(dithiocarbamate)), are present in aparticular molar ratio to each other in the first aqueous solution. Thefirst aqueous solution preferably comprises alkylenediamine,N-aminoalkyldithiocarbamate and alkylene-N,N′-bis(dithiocarbamate) in amolar ratio of 1:0.5:0.5 to 1:10:10, in particular 1:1:1 to 1:10:6. Thefree amino functions of the alkylenediamine or of its reaction productswith 1 mol of carbon disulfide can be in protonated form; as a rule, thedithiocarbamate functions are present as N-methylammonium salt or asinternal salt together with an ammonium group present within the samemolecule. The molar ratio of alkylenediamine and its reaction productswith carbon disulfide is preferably determined indirectly by analyzingthe products obtained after the reaction of the first and the secondaqueous solutions. Thus, an alkylenediamine of the formula I whereR¹═R²═H reacts with 2 moles of N-methyldithiocarbamate and 4 moles offormaldehyde to give (1), N-aminoalkyldithiocarbamate with 1 mol ofN-methyldithiocarbamate, 1 mol of methylammonium ions and 4 moles offormaldehyde to give (2), and alkylene-N,N′-bis(dithiocarbamate) with 2moles of N-methylammonium ions and 4 moles of formaldehyde to give (3).

[0030] The products (1), (2) and (3) can suitably be separated from theproduct obtained (in addition to the main componenttetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione) and determinedquantitatively by means of high-performance liquid chromatography.

[0031] The molar ratio of alkylenediamine and its reaction products withcarbon disulfide in the first aqueous solution can be controlled byvarying the sequence in which methylamine, carbon disulfide andalkylenediamine are combined and/or by varying the residence time priorto addition of a further reactant. Thus, to prepare the first aqueoussolution, it is preferred first to react an aqueous solution ofmethylamine with carbon disulfide and to add the alkylenediamine to theresulting solution. Preferably, 60 to 95% of the reaction betweenmethylamine and carbon disulfide has taken place at the point in timewhen the alkylenediamine is added. The reaction between methylamine andcarbon disulfide can be monitored for example by sampling, monitoringthe pH or monitoring the reaction enthalpy. When using a cascade ofreactors for the preparation of the first aqueous solution, thealkylenediamine is preferably metered into the secondary reactor inorder to adjust the abovementioned ratio between alkylenediamine and itsreaction products with carbon disulfide.

[0032] It has proved advantageous to carry out the reaction of the firstand the second aqueous solution in the presence of small amounts ofelectrolyte. This is achieved expediently by using not demineralizedwater, but tap water or river water, for preparing the first aqueoussolution or for diluting the reaction mixture. The presence of smallamounts of electrolyte presumably prevents electrostatic charging andagglomeration of the precipitated particles. In general, suitableamounts of electrolyte are those which correspond to a conductivity of500 to 1 000 μS/cm.

[0033] When carrying out the process according to the invention and/orpreparing the methylammonium N-methylthiocarbamate solution, waste airis generally obtained which is contaminated with carbon disulfide, andofficial regulations and the like stipulate that this waste air cannotsimply be released into the environment. The waste air loaded withcarbon disulfide can be freed from carbon disulfide by adsorption ontosuitable adsorbents such as active charcoal, or washing with basicliquids, for example aqueous sodium hydroxide or primary, secondary ortertiary amines. In a preferred embodiment, the waste gas is scrubbedwith an aqueous methylamine solution, during which processmethylammonium N-methyldithiocarbamate forms while eliminating most ofthe carbon disulfide from the waste air. The methylamine solution, whichis loaded with some carbon disulfide, can then advantageously be usedfor preparing the first aqueous solution in the process according to theinvention. Waste gas scrubbing with the methylamine solution used asstarting material in the process according to the invention ispreferably employed in a continuous process.

[0034] As an alternative, or additionally, waste gas scrubbing can becarried out with an alkylenediamine, either in the form of an aqueoussolution or, if the alkylenediamine is sufficiently fluid at thetreatment temperature, in substance. In accordance with a preferredembodiment of the process according to the invention, thealkylenediamine obtained during this process, which is loaded with somecarbon disulfide, is then advantageously used for preparing the firstaqueous solution comprising methylammonium N-methylthiocarbamate, as hasbeen described above.

[0035] Traces of iron ions, which originate, for example, from corrosionof the containers used or which are present in the process water, canlead to undesired discolorations of the resultingtetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione. The color may belightened by addition of chelating agents such as nitrilotriacetic acid,ethylenediaminetetraacetic acid,N-(2-hydroxyethyl)ethylenediaminetriacetic acid,diethylenetriaminepentaacetic acid, all of which can be employed in theform of the free acid or as the alkali metal salt, preferably the sodiumsalt. Addition of the complexing agent can take place at any desiredpoint of the process according to the invention or during the reactionof methylamine with carbon disulfide, preferably during the reaction ofcarbon disulfide and methylamine. Suitable amounts are, for example,0.05 to 0.5% by weight, based on the weight of thetetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione formed.

[0036] The invention is illustrated in greater detail by the examplesand comparative examples which follow.

COMPARATIVE EXAMPLE A Batchwise Precipitation

[0037] 2530.1 g of water, 2476.8 g of 40% strength aqueousmonomethylamine, 20.7 g of ethylenediamine and 1540.5 g of CS₂ werereacted per hour at 35° C. in a cascade composed of two 4 l reactorslinked to each other via a barometric discharge. The suspension obtainedwas passed through a filter cartridge operated as coalescence filter.Following phase separation of excess CS₂, this gave 6351.8 g ofmethylammonium N-methyldithiocarbamate as 36.5% strength aqueoussolution (residual CS₂ content approx. 0.05% by weight) per hour.

[0038] A 160 l reactor was charged, at 20° C., with 79.8 kg of water,26.61 kg of 40% strength formaldehyde solution and 1.42 kg of seedmaterial (mean particle size <50 μm), and a total of 61 kg ofmethylammonium N-methyldithiocarbamate were metered in in the course of2 hours as 36.5% strength aqueous solution. After filtration, 27.4 kg ofdazomet were obtained. Particle sizes: 42.0% < 100 μm 98.4% < 200 μm99.6% < 300 μm 99.8% < 400 μm Bulk density: 0.69 kg/l L value (UV-VIS):63.1 (determined by reflectometry, where 100 = complete reflection; 0 =complete absorption of standard light)

EXAMPLE B Continuous Precipitation Without Seeding

[0039] 1686.7 g of water, 1651.2 g of 40% strength aqueousmonomethylamine, 11.1 g of ethylenediamine and 1027.0 g of CS₂ werereacted per hour at 35° C. in a cascade composed of two 4 l reactorslinked to each other via a barometric discharge, the ethylenediaminebeing metered into the second reactor. The suspension was passed througha filter cartridge operated as coalescence filter. After phaseseparation of excess CS₂, this gave 4234.5 g of methylammoniumN-methyldithiocarbamate per hour as 36.5% strength aqueous solution(residual CS₂ content approx. 0.05% by weight).

[0040] 2301.2 g of methylammonium N-methyldithiocarbamate as 36.5%strength aqueous solution, 993.0 g of 40% strength formaldehyde solutionand 3078.7 g of recirculated mother liquor were metered per hour at 25°C. into a 7 l reactor. Centrifugation gave 1146.7 g of dazomet withresidual moisture. Particle sizes:  0.1% < 100 μm 25.6% < 200 μm 91.6% <300 μm 96.4% < 400 μm Bulk density : 0.49 kg/l The above compounds (1),(2) and (3) (where A = 1,2-ethylene) are present in the ratio 33:34:33.L value (UV-VIS): 77.0

[0041] Addition of 3 g of the disodium salt ofethylenediaminetetraacetic acid per hour into the reaction ofmethylamine and carbon disulfide gave dazomet with a color value of92.3.

EXAMPLE C Continuous Precipitation with Seeding

[0042] 57.7 kg of methylammonium N-methyldithiocarbamate as 36.5%strength aqueous solution prepared as described in Example B, 28.1 kg of40% strength formaldehyde solution, 83.6 kg of water and 1.0 kg of seedmaterial (mean particle size <50 μm) were reacted per hour in a 160 lreactor. The resulting dazomet had the following properties: Particlesizes:  6.8% < 100 μm 73.6% < 200 μm 95.6% < 300 μm 98.6% < 400 μm Bulkdensity: 0.68 kg/l

EXAMPLE D Continuous Precipitation with Seeding

[0043] 28.85 kg of methylammonium N-methyldithiocarbamate as 36.5%strength aqueous solution prepared as described in Example B, 14.05 kgof 40% strength formaldehyde solution, 41.8 kg of water and 0.1 kg ofseed material (mean particle size <50 μm) were reacted per hour in a 160l reactor. The resulting dazomet had the following properties: Particlesizes:   2% < 100 μm 50.4% < 200 μm 93.2% < 300 μm 96.8% < 400 μm

We claim:
 1. A process for the preparation of a particulatetetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione product by combininga first aqueous solution comprising methylammoniumN-methyldithiocarbamate with a second aqueous solution comprisingformaldehyde, followed by separation and drying of the resulting solid,which comprises combining the first and the second aqueous solutions insuch a way that the ratio between the concentrations of dithiocarbamatefunctions and of formaldehyde is essentially constant in the reactionmixture over time during the duration of the reaction, and separatingthe solid from the resulting suspension.
 2. The process as claimed inclaim 1, wherein the mass flows of the first and of the second aqueoussolution which are introduced simultaneously into the reaction space aresuch that stoichiometrically essentially equivalent quantities ofdithiocarbamate functions and formaldehyde are introduced per unit time.3. The process as claimed in claim 1 or 2, wherein the first and thesecond aqueous solutions are combined in the presence of finely dividedtetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione.
 4. The process asclaimed in any of the preceding claims, wherein the first aqueoussolution additionally comprises an alkylenediamine and/or reactionproducts thereof with carbon disulfide.
 5. The process as claimed inclaim 4, wherein the first aqueous solution comprises alkylenediamine,N-aminoalkyldithiocarbamate and alkylene-N,N′-bis(dithiocarbamate) in amolar ratio of 1:0.5:0.5 to 1:10:10.
 6. The process as claimed in any ofthe preceding claims, wherein the first aqueous solution is obtained byreacting an aqueous solution of methylamine with a stoichiometric excessof carbon disulfide and removing the unreacted carbon disulfide to lessthan 0.5% by weight based on the aqueous solution.
 7. The process asclaimed in claim 6, wherein the aqueous solution for removing theunreacted carbon disulfide is passed through an apparatus withcoalescence-enhancing elements and the coalesced carbon disulfide isphase separated off.
 8. The process as claimed in any of the precedingclaims, wherein the first and the second aqueous solutions are combinedin the presence of a chelating agent for iron ions.
 9. The process asclaimed in any of claims 6 to 8, wherein the aqueous N-methylaminesolution used is an aqueous methylamine solution loaded with some carbondisulfide, which solution is obtained when the process waste air, whichcontains carbon disulfide, is scrubbed with an aqueous methylaminesolution.
 10. The process as claimed in any of claims 4 to 9, whereinthe alkylenediamine used is an alkylenediamine loaded with some carbondisulfide, which alkylenediamine is obtained when the process waste air,which contains carbon disulfide, is scrubbed with an alkylenediamine oran aqueous solution thereof.